This invention relates to a device for introducing charge and, more particularly, relates to a device for introducing a known amount of charge to a conductor-insulator-semiconductor charge storage element in a charge-coupled device.
Charge-coupled devices provide new ways to accomplish objectives that previously were accomplished by electromechanical and electronic devices. The concepts and applications are set forth in the article, "Charge-Coupled Devices" by Gilbert G. Amelio, Scientific American, vol. 230, No. 2, Feb. 1974. The practicability of these devices was previously established in an article entitled "Experimental Verification of the Charge Coupled Device concept" by Amelio et al, Bell System Technical Journal, Apr. 19, 1970, p. 593.
A necessary feature of any practical digital charge-coupled device is that a known amount of input charge must be written into the first charge storage element. Typically, the introduction of a known amount of charge is used to signify a digital 1 while the absence of charge is used to signify a digital 0 but the inverse convention or any suitable convention may be employed. Existing input writing schemes have required either pulsing a source diffusion, pulsing a control electrode, or performing a combination of these steps. Pulsing a source diffusion requires that the potential of the diffusion region be varied between a high level, V.sub.1, and a low level, V.sub.2. The two levels are specified so that the low potential will spill over a gate electrode and precisely fill the first charge storage element when its clock pulse is at a high potential (in a p-substrate system, charge packets of electrons are transferred). This pulsing scheme requires the pulse level to by precisely controlled and does not readily permit a small amount of charge, sometimes called a "fat zero," to be introduced unless precise and complex pulse timing is employed. If a control electrode is pulsed at the beginning of a line of charge storage elements in a charge-coupled device (CCD line), charge can be stored beneath the control electrode whenever it goes high enough to create a potential well which will receive charge from an adjacent source region. Thus, the logic which drives the control electrode determines whether or not charge is temporarily stored beneath the control electrode. As the potential on the first charge storage element CCD line goes high enough to accept the charge beneath the control electrode, a charge is effectively written into CCD line. In a digital system, the control electrode will have two possible potentials. One potential is lower than that in the source region; the other potential is higher than that in the source region. Thus, a small charge, i.e., a fat zero, cannot be written into the CCD line unless very precise timing of the pulsing of the control electrode is achieved. Also, charge may not be written selectively into addressed lines in a random-access memory incorporating a one-and-one-half phase CCD system (a system with a series of dynamic electrodes in regular alternation with a series of static electrodes) since the clocking pulse on the dynamic electrodes is high in the unaddressed lines of most preferred embodiments and charge cannot readily be kept out of these unaddressed lines.